Improvement of fuel efficiency of tires has been strongly demanded and the improvement of fuel efficiency by improving a rubber composition which composes a tire or by using a lighter tire has been proposed. With respect to a component ratio of raw materials of a pneumatic tire, cords including a steel cord, a bead wire, a resin cord and the like account for the second largest ratio next to a rubber composition rubber component and compounding agents) and therefore, an attempt of improving fuel efficiency of tires by improving these cords, which results in lightening of a tire, has been made.
Since a steel cord and a bead wire are excellent in elastic modulus, strength, initial modulus, heat resistance, dimension stability and the like, these are used as a reinforcing material of a tire. For example, a steel cord is embedded in a belt and the like of a pneumatic tire as a reinforcing material and greatly contributes to secure the strength and shape stability of a tire. Further, by using a bead wire as a bead core of a pneumatic tire, a bead with enough strength necessary for a bead portion and with enough elasticity that can cope with deformation at rim assembling can be obtained.
Methods of lightening cord such as replacing a steel cord or a bead wire with a CFRP (carbon fiber reinforced resin) have been proposed as a method of lightening a tire while maintaining properties such as strength. However, traditionally used CFRPs, which are produced by impregnating a carbon fiber with a matrix resin consisting of an epoxy resin and then curing an impregnated carbon fiber, have a very high elastic modulus. Therefore, there is a problem that a break due to bending easily arises. Particularly, if a bead wire is replaced by a CFRP which is easily subject to a break due to bending, there is a problem that it cannot cope with deformation at rim assembling and a break arises.
As a method of solving the above problems, Patent Document 1 describes that durability and steering stability are improved by using a belt component containing a carbon fiber and a matrix phase consisting of a cured liquid thermosetting resin or a cured liquid rubber in a specified volume ratio. However, neither a combined use of an epoxy resin and a functional group-modified acrylonitrile butadiene copolymer nor fuel efficiency has been considered.
Patent Document 2 disclose a tire having a first ply in which a carbon fiber cord is coated with a coating rubber but not an epoxy resin. However, there is a problem in durability since an adhesion between the carbon fiber cord and the coating rubber is insufficient and there is also a problem that the effect of improving fuel efficiency by lightening is insufficient.